We can view these conditions as a geometry diagram as seen below. So, we know that $\frac{e}{f}=\frac{3}{4}$ (since $e=a-b=\frac{3}{4} c-\frac{3}{4} d=\frac{3}{4} f$ and we know that $\sqrt{e^{2}+f^{2}}=15$ (since this is $\left.\sqrt{a^{2}+c^{2}}-\sqrt{b^{2}+d^{2}}\right)$. Also, note that $a c+b d-a d-b c=(a-b)(c-d)=e f$. So, solving for $e$ and $f$, we find that $e^{2}+f^{2}=225$, so $16 e^{2}+16 f^{2}=3600$, so $(4 e)^{2}+(4 f)^{2}=3600$, so $(3 f)^{2}+(4 f)^{2}=3600$, so $f^{2}\left(3^{2}+4^{2}\right)=3600$, so $25 f^{2}=3600$, so $f^{2}=144$ and $f=12$. Thus, $e=\frac{3}{4} 12=9$. Therefore, \boldsymbol{e f}=\mathbf{9} * \mathbf{1 2}=\mathbf{1 0 8}$.